This invention relates to a convergence error detecting apparatus for use in a projection-type display system.
Conventionally, such a convergence error is monitored by an observer and compensated by a digital convergence adjusting unit contained in the projection-type display system.
In order to correctly carry out the detection and the compensation of the convergence error, convergence error detecting apparatus have recently been proposed which automatically detect the convergence error and transmit the convergence error information to the digital convergence adjusting unit.
Such convergence error detecting apparatus of the type described are disclosed in Unexamined Japanese Patent Publications No. Sho 56-169984, namely, 169984/1981 and No. Sho 63-272294, namely, 272294/1988.
In the convergence error detecting apparatus mentioned in the referenced patent publications, a convergence adjusting pattern is projected as a projected image on a display panel and is detected as a pickup image by a television or a video camera to calculate convergence error information by monitoring a displacement of the convergence adjusting pattern from reference positions.
However, synchronization should be kept between the projected image and the pickup image. In other words, the television or the video camera must be driven by a synchronization signal which is synchronized with that of the projected image of the projection-type display system. Otherwise, an undesirable brightness variation inevitably appears in the form of a belt-shaped portion locally on the pickup image and is moved vertically on the pickup image. Such appearance of a belt-shaped portion makes it difficult to apply the convergence error detecting apparatus to a projection-type display system which is operable in accordance with scanning frequencies different from those of the television or the video camera. In other words, no consideration is made about a convergence error detecting apparatus which is operable in response to the scanning frequency different from that of the projection-type display systems. If the convergence error detecting apparatus can be driven by a synchronization frequency different from that of the projected image, such a convergence error detecting apparatus can be applied to a wide variety of television signals.